1. Field of the Invention
The present invention relates to a spindle end positional deviation correction system for machine tools, and particularly, it relates to a spindle end positional deviation correction System for compensating a deviation in axial position (Z-axis position) of a tool at a spindle, due such as to a thermal deformation of the spindle, in a machine tool of a numerical control type.
2. Description of Relevant Art
The machine tool generally has its spindle thermally deformed due to heat generation such as at bearings of the spindle as well as a spindle motor, during a long processing operation. In the numerical control machine tool, such a thermal deformation causes a coordinate position relationship between a spindle and a work table to be deviated in a Z-axis direction, resulting in a reduced processing accuracy. The coordinate position relationship between spindle and work table is caused to deviate in Z-axis direction, also by deflections of the spindle and bearings supporting the spindle, due to reaction forces of cutting actions, as well as by a contraction of the spindle due to a centrifugal force of the spindle in rotation, resulting in a reduced processing accuracy.
There have been made attempts over many years to compensate for the reduced processing accuracy in Z-axis direction due to a deformed spindle. In an attempt, body temperatures (such as at a spindle holding ram) of a machine tool were measured by temperature sensors and processed to estimate a quantity of thermal deformation in an axial direction of a spindle, and a (Z-axis) command for a position in the axial direction of the spindle was corrected in dependence on the estimation, to thereby compensate for the thermal deformation quantity in a canceling manner.
As another conventional art relevant to the present invention, there has been disclosed in Japanese Utility Model Application Laid-Open Publication No. (Hei)1-92347 a thermal positional deviation correction device, in which a thermal positional deviation in a radial direction of a spindle was measured by a non-contact type gap sensor provided at a housing end, and a (X-axis) position in the radiation direction of the spindle was corrected in dependence on a measured value of the gap sensor.
In general, the quantity of thermal deformation in an axial direction of a spindle is estimable from machine body temperatures. However, such an estimated quantity does not always represent the quantity of an actual deformation. Therefore, the positional deviation correction in axial direction of spindle may unsuccessfully depend on body temperatures, failing to achieve a spindle end positional deviation correction with a high accuracy.
Further, due to relatively large heat-mass dependent time constants such as of a spindle and a ram, it is difficult to perform the spindle end positional deviation correction with high accuracy, sometimes on a real time basis, and it actually is impossible to do so with a required precision.
The positional correction in axial direction of a spindle, so far as it depends on a machine body temperature, is thus applicable simply to a correction of a positional deviation due to a thermal deformation, and unable to correct positional deviations in axial direction of the spindle due to other causes.
In this respect, the thermal positional deviation correction device disclosed in the Japanese Utility Model Application'Laid-Open Publication No. (Hei)1-92347 is for correction of a radial (X-axis) position of a spindle, but not for that of an axial (Z-axis) position off spindle.